1. Technical Field
The invention relates to multi-coil units, voice coils, and electro-acoustic transducers that are applicable to digital speakers and other apparatuses.
2. Background Art
Developments have been made on digital speakers that directly receive and reproduce digital audio signals, instead of converting digital signals into analog. A known digital speaker of this type has a plurality of coils wound around a voice coil bobbin. The coils are weighted in such a manner to generate respective driving forces corresponding to respective bits of a digital signal. Each coil carries a predetermined voltage, the polarity of which is switched in accordance with a 2-bit binary value of a digital signal, thereby determining the direction of current flowing through the coil in accordance with the binary value. This configuration allows the digital speaker to generate driving forces at a ratio corresponding to the quantization of a digital signal (see Japanese Unexamined Patent Application Publication No. 2010-263332 A).
Such a digital speaker may have a voice coil configured as shown in FIG. 5. The reference numeral 13 in FIG. 5 denotes a voice coil, 1 denotes a yoke, 2 denotes a magnet, 3 denotes a pole piece, and G denotes a magnetic gap. The voice coil 13 includes a coil 13A, a coil 13B, and a coil 13C in accordance with the number of quantization bits of a digital signal. The coil 13A has a winding structure in which a coil wire is wound a plurality of times in a coil vibration direction (α in FIG. 5) to be stacked (in two tiers in the example of FIG. 5) in a magnetic flux direction (β in FIG. 5). The same holds true for the coil 13B and the coil 13C. The coil 13B is wound on the outer periphery of the coil 13C, and the coil 13A is wound on the outer periphery of the coil 13B.